The present invention relates to an improved perpendicular magnetic recording medium comprising a sputtered multilayer magnetic superlattice exhibiting very high values of perpendicular magnetic coercivity and areal storage density, and a method for manufacturing same. The invention finds particular utility in the fabrication of very high areal recording density magnetic recording (xe2x80x9cMRxe2x80x9d) media and devices such as hard disks.
Magnetic recording media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval purposes, typically in disk form. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as xe2x80x9clongitudinalxe2x80x9d or xe2x80x9cperpendicularxe2x80x9d, depending upon the orientation of the magnetic domains of the grains of magnetic material.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, of the magnetic media. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits. In this regard, the perpendicular recording media have been found superior to the more common longitudinal media in achieving very high bit densities.
As indicated above, much effort has been directed toward enhancing the density of data storage by both types of magnetic media, as well as toward increasing the stability of the stored data and the ease with which the stored data can be read. For example, it is desirable to develop magnetic media having large magnetic coercivities, Hc, since the magnetic moments of such materials require large magnetic fields for reorientation, i.e., switching between digital 1 and 0. Thus, when the magnetic medium has a large coercivity Hc, exposure of the magnetic medium to stray magnetic fields, such as are generated during writing operations, is less likely to corrupt data stored at adjacent locations.
The density with which data can be stored within a magnetic thin-film medium for perpendicular recording is related to the perpendicular anisotropy (xe2x80x9cKuxe2x80x9d or xe2x80x9cKxe2x8axa5xe2x80x9d) of the material, which reflects the tendency for the magnetic moments to align in the out-of-plane direction. Thin-film magnetic media having high perpendicular anisotropies have their magnetic moments aligned preferentially perpendicular to the plane of the thin film. This reduces the transition length, thereby allowing a larger number of magnetic bits (domains) to be packed into a unit area of the film and increasing the areal density with which data can be stored.
A large perpendicular anisotropy is also reflected in a larger value of the magnetic coercivity Hc, since the preferential out-of-plane alignment of the magnetic moments raises the energy barrier for the nucleation of a reverse magnetization domain, and similarly, makes it harder to reverse the orientation of the magnetic domains by 180xc2x0 rotation. Further, the magnetic remanence of a medium, which measures the tendency of the magnetic moments of the medium to remain aligned once the magnetic field is shut off following saturation, also increases with increasing Kxe2x8axa5.
A promising new class of materials for use as the active recording layer of perpendicular MR media includes multilayer magnetic xe2x80x9csuperlatticexe2x80x9d structures comprised of a stacked plurality of very thin magnetic/non-magnetic layer pairs, for example cobalt/platinum (Co/Pt)n and cobalt/palladium (Co/Pd)n multilayer stacks. As schematically illustrated in the cross-sectional view of FIG. 1, such multilayer stacks or superlattice structures 10 comprise n pairs of alternating discrete layers of Co (designated by the letter A in the drawing) and Pt or Pd (designated by the letter B in the drawing), where n is an integer between about 5 and about 50. Superlattice 10 is typically formed by a suitable thin film deposition technique, e.g., sputtering, and can exhibit perpendicular magnetic isotropy arising from metastable chemical modulation in the direction normal to the underlying substrate S on which superlattice 10 is formed. Compared to conventional cobalt-chromium (Co-Cr) magnetic alloys utilized in magnetic data storage/retrieval disk applications, such Co/Pt)n and (Co/Pd)n, multilayer magnetic superlattice structures offer a number of performance advantages. For example, a sputtered (Co/Pt)n multilayer stack or superlattice 10 suitable for use as a magnetic recording layer of a perpendicular MR medium can comprise n Co/Pt or Co/Pd layer pairs, where n=about 5 to about 50, e.g., 20, and wherein each Co/Pt layer pair consists of an about 3 xc3x85 thick Co layer adjacent to an about 10 xc3x85 thick Pt or Pd layer, for a total of 40 separate (or discrete) layers. Such multilayer magnetic superlattice structures are characterized by having a large perpendicular anisotropy, high coercivity Hc, and a high squareness ratio of a magnetic hysteresis (M-H) loop measured in the perpendicular direction. By way of illustration, (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattices, wherein n=about 10 to about 30 Co/Pt or Co/Pd layer pairs having thicknesses as indicated supra and fabricated, e.g., by means of techniques disclosed in U.S. Pat. No. 5,750,270, the entire disclosure of which is incorporated herein by reference, exhibit perpendicular anisotropies exceeding about 2xc3x97106 erg/cm3; coercivities as high as about 5,000 Oe; squareness ratios (S) of a M-H loop, measured in the perpendicular direction, of from about 0.85 to about 1.0, and carrier-to-noise ratios (xe2x80x9cCNRxe2x80x9d) of from about 30 dB to about 60 dB.
A key advance in magnetic recording (MR) technology which has brought about very significant increases in the data storage densities of magnetic disks has been the development of extremely sensitive magnetic read/write devices which utilize separate magnetoresistive read heads and inductive write heads. The magnetoresistive effect, wherein a change in electrical resistance is exhibited in the presence of a magnetic field, has long been known; however, utilization of the effect in practical MR devices was limited by the very small magnetoresistive response of the available materials. The development in recent years of materials and techniques (e.g., sputtering) for producing materials which exhibit much larger magnetoresistive responses, such as Fexe2x80x94Cr multilayer thin films, has resulted in the formation of practical read heads based upon what is termed the giant magnetoresistive effect, or xe2x80x9cGMRxe2x80x9d. Further developments in GMR-based technology have resulted in the formation of GMR-based head structures, known as GMR xe2x80x9cspin valvexe2x80x9d heads, which advantageously do not require a strong external magnet or magnetic field to produce large resistance changes, and can detect weak signals from tiny magnetic bits.
The use of such GMR-based spin valve heads can significantly increase the areal density of MR media and systems by increasing the track density, as expressed by the number of tracks per inch (xe2x80x9cTPIxe2x80x9d) and the linear density, as expressed by the number of bits per inch (xe2x80x9cBPIxe2x80x9d), where areal density=TPIxc3x97BPI. Currently, GMR-based spin valve heads are utilized for obtaining areal recording densities of more than about 10 Gb/in2; however, even greater recording densities are desired. A difficulty encountered with further increase in the BPI of conventional MR media is that the smaller grain sizes necessary for increase in the BPI results in thermal instability of the media due to exceeding the superparamagnetic limit.
As indicated supra, sputtered multilayer magnetic superlattice structures can provide several advantages vis-à-vis conventional thin-film MR media, when utilized in fabricating very high areal density MR media. Specifically, multilayer magnetic superlattice MR media exhibit higher interfacial (i.e., perpendicular) anisotropy (Ku or Kxe2x8axa5) than conventional thin film MR media, e.g., greater values of KuV/kT (where Ku=anisotropy constant; V=volume in cm3; k=Boltzmann""s constant; and T=absolute temperature, xc2x0 K.); increased thermal stability; and very high values of perpendicular coercivity Hc, i.e.,  greater than 103 Oe. In this regard, the very high values of Hc attainable with multilayer magnetic superlattice structures translates into a significant increase in BPI, and thus a substantial increase in areal recording density.
Multilayer magnetic superlattice structures utilized in MR media can have either an ordered structure forming a true superlattice, or a disordered structure, variously termed a xe2x80x9cnon-superlattice multilayerxe2x80x9d or a xe2x80x9cpseudo-superlattice structurexe2x80x9d. In (Co/Pt)n and (Co/Pd)n multilayer superlattice structures having utility in high areal density MR media, the interfaces between the Co and Pt (or Pd) layers incur surface interactions, such as for example, spin-orbit coupling. Because Co and Pt (or Pd) have different electronic shell structures or configurations, spin-orbit coupling occurs between the spin and orbital motions of the electrons. When an external magnetic field is applied for reorienting the spin of an orbiting electron, the orbit is also reoriented. However, because the orbit is strongly coupled to the metal lattice, the spin axis of the electron resists rotation. The energy required for reorienting (i.e., rotating) the spin system of a magnetic domain away from the easy direction is termed the anisotropy energy. The stronger the spin-orbit coupling or interaction, the higher the anisotropy energy and the coercivity Hc.
It is believed that an increase in the interfacial anisotropy of (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattice structures, as by an increase in the disorder or xe2x80x9cbroken symmetryxe2x80x9d at each of the Co/Pt or Co/Pd interfaces, can result in the obtainment of very high perpendicular magnetic coercivities Hc necessary for fabricating ultra-high areal density, thermally stable MR media, i.e., perpendicular coercivities on the order of about 104 Oe. It is further believed that the amount or degree of disorder or xe2x80x9cbroken symmetryxe2x80x9d at each of the Co/Pt or Co/Pd interfaces of sputtered (Co/Pt)n and (Co/Pd)n superlattices can be increased by substituting the conventionally employed Ar sputtering gas with a higher atomic weight sputtering gas, e.g., Kr or Xe, thereby providing bombardment of the deposited films with ionized particles having greater momentum, resulting in a greater amount of lattice disruption, disorder, and/or xe2x80x9cbroken symmetryxe2x80x9d at the layer interfaces, as disclosed in U.S. Pat. No. 5,106,703 to Carcia, the entire disclosure of which is incorporated herein by reference.
Referring again to FIG. 1, according to conventional practices for forming multilayer magnetic superlattice structures 10 in the fabrication of magnetic recording (MR) media, at least one thin underlayer, termed a xe2x80x9cseed layerxe2x80x9d, typically comprising at least one material selected from Pt, Pd, Ag, Au, Rh, Ir, Cu, and Mn, is interposed between the substrate S and the multilayer magnetic superlattice structure 10 for enhancing the crystallographic texturing and controlling the grain size/structure of the latter.
A frequently encountered problem in the preparation of multilayer magnetic superlattice structures for use in large scale, automated fabrication of very high recording density magnetic media is difficulty in reliably and controllably achieving a desired interfacial anisotropy of the magnetic superlattice, hence a desired perpendicular magnetic coercivity Hc.
Accordingly, there exists a need for improved methodology for forming, by sputtering, (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattice structures exhibiting very high and readily controllable perpendicular magnetic coercivities Hc, i.e., as high as about 5,000 Oe, by means and methodology which can be easily and readily implemented in a cost-effective manner for fabrication of very high areal recording density MR media. Further, there exists a need for improved perpendicular MR media having very high areal recording densities and improved thermal stability, which media can be fabricated in an economical fashion utilizing conventional automated manufacturing equipment.
The present invention is based upon the discovery that regulation and control of the perpendicular magnetic coercivity of multilayer magnetic superlattice structures such as described supra can be accomplished by use of ultra-thin seed or underlayers and by proper selection of the thickness of the ultra-thin seed or underlayer, such that the seed layer thickness determines the extent of a domain wall pinning effect exerted on the magnetic superlattice, whereby the perpendicular anisotropy, hence perpendicular magnetic coercivity Hc, is substantially enhanced and well-controlled. According to the present invention, (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattice structures having very large values of perpendicular coercivities Hc as high as about 5,000 Oe, are formed over improved, ultra-thin seed or underlayers having a thickness up to about 30 xc3x85 by sputtering of Co and Pt or Pd targets at sputtering gas pressures of at least about 30 mTorr. The inventive methodology thus effectively addresses and solves problems attendant upon reliably and controllably obtaining high quality multilayer magnetic superlattice structures suitable for use in manufacturing improved perpendicular MR media having very high areal recording densities with good thermal stability, while maintaining full compatibility with all aspects of automated MR media manufacturing technology. Further, the methodology provided by the present invention enjoys diverse utility in the manufacture of all manner of films, devices, and products requiring multilayer magnetic thin film coatings and structures exhibiting very high values of perpendicular anisotropy and magnetic coercivity.
An advantage of the present invention is an improved method for reliably and controllably forming magnetic thin-film, multilayer magnetic superlattice stacks and structure having predetermined high perpendicular magnetic coercivities Hc.
Another advantage of the present invention is an improved method for reliably and controllably manufacturing very high recording density perpendicular magnetic recording (MR) media including improved, ultra-thin seed or underlayers and multilayer magnetic superlattice structures having perpendicular magnetic coercivities as high as about 6,000 Oe.
Yet another advantage of the present invention is improved very high recording density, perpendicular magnetic recording media having improved, ultra-thin seed or underlayers and multilayer magnetic superlattice structures having perpendicular magnetic coercivities as high as about 6,000 Oe.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are obtained in part by a method of forming a magnetic thin film having a multilayer magnetic superlattice structure, the multilayer magnetic superlattice structure having a predetermined perpendicular magnetic coercivity Hc and comprising a stacked plurality of magnetic/non-magnetic layer pairs, the method comprising the steps of:
(a) providing a non-magnetic substrate having a surface;
(b) forming an ultra-thin seed layer of predetermined thickness over the substrate surface; and
(c) forming the multilayer magnetic superlattice structure on the ultra-thin seed layer by sputtering in an atmosphere having at least a predetermined minimum pressure;
wherein the predetermined perpendicular magnetic coercivity Hc of the multilayer magnetic superlattice structure formed in step (c) is determined by the predetermined thickness of the seed layer formed in step (b).
According to embodiments of the present invention, step (b) comprises forming, e.g., by sputtering over the substrate surface, an ultra-thin seed layer comprising at least one material selected from the group consisting of Pd, Pt, a Pd/Pt bi-layer, a Pt/Pd bi-layer, metals, semi-metals, non-metals, oxides, nitrides, and combinations thereof; and step (c) comprises forming the multilayer magnetic superlattice by alternately sputter depositing the magnetic and non-magnetic layers of the stacked plurality of layer pairs in a sputtering gas atmosphere having at least a minimum pressure of about 30 mTorr.
According to particular embodiments of the invention, step (b) comprises sputter depositing an ultra-thin seed layer of Pd having a predetermined thickness up to about 30 xc3x85; and step (c) comprises forming the stacked plurality of magnetic/non-magnetic layer pairs such that each magnetic layer is from about 2 to about 10 xc3x85 thick and comprises a material selected from the group consisting of Co, CoCr, and CoX, where X is B, Ru, Ta, Pt, or Pd, each non-magnetic layer is from about 3 to about 20 xc3x85 thick and comprises a material selected from the group consisting of Pt and Pd, and the number n of magnetic/non-magnetic layer pairs is from about 5 to about 50. According to a specific embodiment, step (c) comprises forming a (Co/Pd)n or a (Co/Pt)n multilayer magnetic superlattice structure, wherein each Co layer is about 3 xc3x85 thick, each Pd or Pt layer is about 10 xc3x85 thick, n=20, and the predetermined magnetic coercivity Hc is in the range of from about 3,000 to about 6,000 Oe.
According to a further aspect of the present invention utilizing the above sequence of steps in a method for manufacturing a perpendicular magnetic recording medium including a multilayer magnetic superlattice structure, step (a) comprises providing a non-magnetic substrate comprising a non-magnetic material selected from the group consisting of non-magnetic metals, non-magnetic metal alloys, Al, Al-based alloys, NiP-plated Al, glass, ceramics, polymers, composites, and laminates thereof, the substrate further including a layer of a high magnetic moment material from about 2,000 to about 5,000 xc3x85 thick on the substrate surface and comprised of a material selected from the group consisting of FeAlN, NiFe, CoNiFe, and CoZrNb.
According to further embodiments of the present invention, the method comprises the further steps of:
(d) providing a protective overcoat layer on the multilayer magnetic superlattice structure; and
(e) providing a lubricant topcoat layer over the protective overcoat layer.
According to particular embodiments of the invention, step (d) comprises depositing a protective overcoat layer having a thickness of from about 20 to about 100 xc3x85 thick and comprised of a wear-resistant material selected from the group consisting of diamond-like carbon (xe2x80x9cDLCxe2x80x9d), a-C:H, a-CHxNy, a-C:N, ion beam-deposited carbon (xe2x80x9cIBD-Cxe2x80x9d), cathodic arc-deposited carbon (xe2x80x9cCAD-Cxe2x80x9d), SiNx, AlNx, SiC, SiN/C, AlN/C, and SiC/C; and step (e) comprises applying a lubricant topcoat having a thickness of from about 10 to about 35 xc3x85 and comprised of a high molecular weight fluoropolyether or perfluoropolyether.
According to another aspect of the present invention, a perpendicular magnetic recording medium comprises, in sequence:
(a) a non-magnetic substrate having a surface;
(b) a layer of a high magnetic moment material on the substrate surface;
(c) an ultra-thin seed layer up to about 30 xc3x85 thick on the layer of high magnetic moment material; and
(d) a multilayer magnetic superlattice structure on the seed layer, the multilayer magnetic superlattice structure comprising a stacked plurality of magnetic/non-magnetic layer pairs and having a perpendicular magnetic coercivity Hc of from about 3,000 to about 6,000 Oe.
According to embodiments of the present invention, the non-magnetic substrate (a) comprises a non-magnetic material selected from the group consisting of non-magnetic metals, non-magnetic metal alloys, Al, Al-based alloys, NiP-plated Al, glass, ceramics, polymers, composites and laminates thereof; layer (b) of high magnetic moment material is from about 2,000 to about 5,000 xc3x85 thick and comprises a material selected from the group consisting of FeAlN, NiFe, CoNiFe, and CoZrNb; ultra-thin seed layer (c) comprises at least one material selected from the group consisting of Pd, Pt, a Pd/Pt bi-layer, a Pt/Pd bi-layer, metals, semi-metals, non-metals, oxides, nitrides, and combinations thereof; and the multilayer magnetic superlattice structure (d) comprises n magnetic/non-magnetic layer pairs, wherein n is an integer from about 5 to about 50, the magnetic layer of each layer pair is from about 2 to about 10 xc3x85 thick and comprises a material selected from the group consisting of Co, CoCr, and CoX, where X is B, Ru, Ta, Pt, or Pd, and the non-magnetic layer of each layer pair is from about 3 to about 20 xc3x85 thick and comprises a material selected from Pt and Pd.
According to particular embodiments of the present invention, the seed layer (c) comprises sputtered Pd; and the multilayer magnetic superlattice structure comprises sputtered (Co/Pd)n or (Co/Pt)n, wherein each Co layer is about 3 xc3x85 thick, each Pd or Pt layer is about 10 xc3x85 thick, and n=20.
According to further embodiments of the present invention, the medium further comprises:
(e) a protective overcoat layer on the multilayer magnetic superlattice structure; and
(f) a lubricant topcoat layer on the protective overcoat layer.
According to specific embodiments of the present invention, the protective overcoat layer (e) is from about 20 to about 100 xc3x85 thick and comprises a wear-resistant material selected from the group consisting of diamond-like carbon (xe2x80x9cDLCxe2x80x9d), a-C:H, a-CHxNy, a-C:N, ion beam-deposited carbon (xe2x80x9cIBD-Cxe2x80x9d), cathodic arc-deposited carbon (xe2x80x9cCAD-Cxe2x80x9d), SiNx, AlNx, SiC, SiN/C, AlN/C, and SiC/C; and the lubricant topcoat layer (f) is from about 10 to about 35 xc3x85 thick and comprises a high molecular weight fluoropolyether or perfluoropolyether.
Still another aspect of the present invention is a perpendicular magnetic recording medium comprising:
a multilayer magnetic superlattice structure formed over a surface of a non-magnetic substrate; and
ultra-thin seed layer means intermediate the multilayer magnetic superlattice structure and the substrate surface for providing the medium with a predetermined perpendicular magnetic coercivity Hc of from about 3,000 to about 6,000 Oe.